泥石流多出现在[ ]

A．地势平坦的平原          

B．植被覆盖好的高原    

C．坡陡雨多的山地          

D．气候干燥的沙漠

Vapor given off by a flammable liquid will burn when ignited provided（）

A.the vapor and air mixture is too rich

B.it is mixed with certain proportions of air

C.the vapor is too little compared to the air

D.it is of heavy toxicity